Fluid purification is an obligatory step for several industrial processes. For example, gas purification typically involves removal of water, carbon dioxide, or other unwanted gases that may interfere with the end use of the purified gas. Industrial gases that need to be purified before use include air, nitrogen, helium, argon, hydrogen, oxygen, and hydrocarbons.
Industrial gases also require careful purification before being released into the atmosphere. The most common contaminants present in these industrial gases are carbon dioxide, sulfur dioxide and trioxide, nitrogen oxides, hydrogen sulfide and small organic molecules. Removal of these impurities is important to reduce environmental pollution and minimize overall climate change. The most commonly used processes to purify gases on an industrial scale are liquid scrubbers (where a basic or acidic solution is used to absorb an acidic or basic gas, respectively), exchange resins (where immobilized bases or acids are used to absorb an acidic or basic gas, respectively), or membranes (which separate gases based on competitive adsorption, differences in diffusion rates, molecular discrimination, and/or sieving).
Separation membranes are likely to play an increasingly important role in reducing the environmental impact and the costs of industrial processes, because their use generates minimal amount of byproducts and has low energy footprint (Baker, 2002, Ind. & Eng. Chem. Res. 41(6):1393-1411; Koros, 2004, AIChE J. 50(10):2326-2334; Noble & Agrawal, 2005, Ind. & Eng. Chem. Res. 44(9):2887-2892). Commercially important gas separations include H2 purification from light gases related to coal gasification, and CO2 removal from CH4 in natural gas processing, with gas molecule size differences ranging from 0.02 nm (O2/N2) to 0.09 nm (H2/CH4). Dense membranes can separate gas mixtures based on competitive adsorption and/or differences in diffusion rates, whereas porous membranes can separate gas mixtures via molecular discrimination or sieving (Wijmans & Baker, 1995, J. Membr. Sci. 107(1-2):1-21; Gin et al., 2008, Macromol. Rapid Comm. 29(5):367-389).
Certain organic polymers have been found to be particularly suitable for producing separation membranes on an industrial scale. Gas permeability in such polymer membranes is dominated by the diffusivity of the gas species throughout the polymer network. As the diffusivity is related to the mobility of gas molecules within the polymer, the differential transportation of gas species throughout a polymer membrane is believed to be dictated by two key parameters. These are (1) the accessible “free volume” of the polymer, and (2) the particular configuration of the pores and channels contributing to that free volume throughout the polymer mass, i.e. the “free-volume distribution”.
A polymer's free volume is defined as the difference between the specific polymer volume in its glassy or rubbery state and the occupied volume associated with the material in its crystalline configuration extrapolated to zero Kelvin. The fractional free volume is the ratio between that difference and the polymer volume in its glassy or rubbery state at the given temperature. The fractional free volume can therefore be expressed in vol. % or volumetric fraction. The fractional free volume is therefore a measure of the residual “voids” that remain between the polymeric chains when these are inter-locked in their 3D arrangements.
On the other hand, the free-volume distribution relates to how the free volume is arranged spatially within the polymer, by way of interconnected porosity and channels. It is the free volume distribution that is of interest in understanding the mechanisms underlying the separation of fluid mixtures, since its configuration will dictate which molecules filters through the polymer and which molecules may remain adsorbed on the surface of the free volume pockets. While two polymers may have the same total free volume, they may have vastly differing transport properties based upon a different free volume distribution.
Ideally, separation membranes should exhibit both high flux and high selectivity.
Polymers suitable for use as separation membranes are generally characterized by fractional free volume values ranging from about 0.1 to about 0.5.
From a thermodynamic point of view, the molecular arrangement of polymer chains giving rise to a detectable free volume is one of non-equilibrium. As a result, such polymers tend to evolve into lower and more stable energy states over time. Consequently, the corresponding free volume tends to correspondingly collapse and diminish. This process is a commonly referred to as “relaxation” or “ageing” of the polymer. In the context of separation membranes, this phenomenon can dramatically affect the available free volume and free volume distribution for gas separation purposes. Indeed, a common problem affecting the performance of separation membranes is their reduced capability to maintain their permeability characteristics over time due to such ageing effects causing a dramatic reduction of the available free volume.
Polymers suitable for use as membranes in separation include polymers of intrinsic microporosity (PIMs), thermally rearranged (TR) polymers, hyperbranched polymers and substituted polyacetylenes.
Substituted polyacetylenes have been used to good effect as separation membranes. Polyacetylene is an organic polymer with the repeating unit (—CH═CH—). The polymer consists of a long chain of carbon atoms with alternating single and double bonds between them, each with one hydrogen atom. In substituted polyacetylenes, functional groups replace one or both of the hydrogen atoms in the repeating unit.
An example of the aforementioned aging phenomenon may be described with reference to poly (1-(trimethylsilyl)-1-propyne) (PTMSP), which is a substituted polyacetylene.
PTMSP is particularly suitable for gas separation applications due to its high fractional free volume. The high gas permeability of PTMSP is attributed to the fact that the polymer displays a large amount of fractional free volume in which the inter-chain void regions are highly interconnected (Srinivasan et al., 1994, J. Membr. Sci. 86(1-2):67-86). This free volume is the result of bulky side groups (trimethylsilyl groups) attached to the rigid polyacetylene backbone. However, because of the non-equilibrium state of the as-synthesized PTMSP, this initial large free volume in the material tends to collapse over time, resulting in a tremendous decrease in gas permeability. The physical “aging” and loss of permeability for PTMSP has been observed in numerous studies. For example, Nagai et al. reported a decrease of the permeability and diffusion coefficient of poly (1-trimethylsilyl-1-propyne-co-1-phenyl-1-propyne) membranes for various gases by 1 to 2 orders of magnitude after only 3 days (Nagai et al., 1995, J. Polym. Sci. Part B: Pol. Physics 33(2):289-298).
This degradation in properties hampers the use of substituted polyacetylenes in industrial applications. Several approaches have been explored to increase or stabilize the initially high gas permeabilities of PTMSP, such as physical blends preparation, polymer cross-linking, copolymer synthesis and functionalization. However, no study has solved the problem of the long-term stabilization of the desired gas permeation properties of any substituted polyacetylene (including PTMSP).
An opportunity therefore remains to develop new polymer compositions suitable for use as fluid separation membranes that exhibit improved permeability properties such as an extended period of time over which permeability is maintained (i.e. membranes that show reduced aging effects). Membranes prepared with such compositions should be useful for fluid separation processes, including but not limited to gas-phase separations.